In order to discharge water containing a metal element out of factories and the like, it is required to remove the residual metal element by some kinds of methods. As a method of removing the metal element from water, there are a coagulating sedimentation method, an ion exchange method, an adsorption method to adsorb the metal element on an adsorbent such as activated carbon, an electrical adsorption method, a magnetic adsorption method, and the like, but a coagulating sedimentation method using a neutralizing agent is frequently used as a general method.
Specifically, as a coagulating sedimentation method, a method is employed in which the metal is solidified as a hydroxide by adding a neutralizing agent to the water to be treated to raise the pH, the solid is then separated from the liquid by an operation such as filtration, the liquid is discharged out of the factory, and the solid is treated at a waste disposal site or the like. An inexpensive calcium-based neutralizing agent such as limestone or slaked lime is generally used as the neutralizing agent to be used in the coagulating sedimentation method.
However, when treating water containing chromium (Cr) as a metal element, it is not possible to sufficiently efficiently and effectively separate chromium by the coagulating sedimentation method described above in some cases.
Specifically, in order to effectively immobilize chromium as a hydroxide, it is required to react chromium after once being reduced from hexavalent chromium to trivalent chromium and thus to conduct the reduction treatment by using a reducing agent, but the use of a reducing agent results in a cost increase.
For such a reason, there is a demand for a method of inexpensively treating water containing chromium.
For example, Patent Document 1 discloses a method of treating chromium-containing wastewater which includes a reduction step of reducing hexavalent chromium to trivalent chromium by adding a ferrous ion to the water containing hexavalent chromium, an insolubilization step of converting the trivalent chromium produced in the reduction step to an insoluble hydroxide by adding an alkali to the effluent from the reduction step, and a sludge separation step of separating the insoluble hydroxide from the effluent from the insolubilization step and in which a part of the sludge separated in the sludge separation step is introduced into the reduction step.
However, according to the technique described in Patent Document 1, it is required to prepare a reducing agent in order to reduce hexavalent chromium to trivalent chromium, and this results in expensive treatment cost.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H07-80478